For decades, researchers have been trying to increase the storage density and reduce the cost/storage in information storage devices, such as magnetic hard-drives, optical drives, and DRAM. However, it has become increasingly difficult to squeeze more information into the storage devices. Moreover, conventional technologies to make those devices may be approaching fundamental limits on storage density.
Many scientists have proposed alternative approaches to increase the storage density. One approach is based on Scanned Probe Microscopy (SPM) technologies. Typically, in such an approach, a probe is positioned extremely close to a storage medium. For example, in one implementation of Atomic Force Microscopy, the probe physically touches the medium; in Scanning Tunnelling Microscopy (STM), the probe is within a few nanometers of the medium to ensure that the probe is within tunnelling range of the medium. It is difficult to inexpensively build a storage system with a probe in contact with or in such extremely close proximity to the medium without, eventually, damaging or effacing the surface of the medium or the probe. Moreover, in STM, the nanometer spacing must be precisely controlled. This is a difficult task. Some researchers have found methods to eliminate the need for such extremely close proximity. One technique is based on Near-Field Scanning Optical Microscopy. However, this technique has limited lateral resolution and bandwidth. Other techniques are based on non-contact Scanning Force Microscopy, which typically suffers from poor resolution and poor signal to noise ratio.
Even if one has increased the storage density, one still has to overcome another major hurdle, which is the time required to access the information. The storage device's utility is limited if it takes a long time to retrieve the stored information. In other words, in addition to high storage density, one must find a way to quickly access the information.
It should be apparent from the foregoing that there is still a need for a storage device that is based on a non-conventional approach, with significantly increased storage density, and low cost/storage. Additionally, the storage device preferably should have fast access times and high data rates. Furthermore, the storage device should preferably eliminate the requirement for extremely close proximity between the probe and storage medium.
U.S. Pat. No. 5,557,596 to Gibson et al. discloses a ultra-high density storage device. The storage device includes many field emitters in close proximity to a storage medium, and a micromover for moving the storage medium relative to the field emitters. In one embodiment, the storage medium uses a heterojunction diode. Information is read from the storage device by generating a signal current from the storage area bombarded by the electron beam current, wherein the magnitude of the signal current depends on the state of the storage area. One known problem with this approach is the difficulty to create a good phase change to silicon heterojunction for device operation.